Cable stranding machines are used in cable manufacturing to form cables with multiple strand elements (“strands”) having an alternating twist direction. Such cables are called “SZ” cables because the strands periodically helically twist in opposing “S” and “Z” directions. The SZ stranding configuration eliminates the need for the strand storage containers to be rotated around the cable core member, thereby resulting in less complex, faster-operating stranding machinery.
The strands, which can be wire, optical fibers, buffer tubes, etc., are stored in storage containers (e.g., spools or “packages”) and pass through a stationary guide or “layplate.” The layplate keeps the strands locally spaced apart as they pass through to a downstream SZ cable-stranding apparatus. Prior art SZ cable-stranding apparatus employ a series of axially arranged and mechanically coupled guides typically in the form of non-stationary (i.e., rotatable) plates called “layplates” similar if not identical to the stationary layplate. The rotatable layplates also serves to keep the strands locally spaced apart during the stranding process to ensure that the strands do not become entangled with each other or the core member as the layplates rotate through their motion profiles.
In the process of forming an SZ-stranded cable, the layplates are mechanically coupled and driven in alternating rotational directions at progressively slower rates towards the upstream stationary plate as the strands move through the layplates. An SZ-stranded assembly, consisting of the strands wound around the central core member, emerges from the most downstream rotatable layplate.
In the simplest form of SZ cable-stranding apparatus, tension in the strands provides the mechanical coupling that rotates the layplates. However, this results in poor tension control with a limited range of layplate rotation. More complex and expensive approaches use a series of shafts from a drive member (“prime mover”) and belts and/or gears to synchronize the motion of the rotating layplates to generate the required rotation rate for each layplate. An example of this type of SZ cable-stranding apparatus uses an elastic shaft running parallel to the axis of the oscillator. The torsion of the shaft, in combination with an arrangement of belts, pulleys and/or gears, drives the layplates.
Generally, mechanically based SZ cable-stranding apparatus are expensive and difficult to maintain. Furthermore, the added rotational inertia of the mechanical components limits the maximum rate at which the rotatable layplates can reverse directions, thereby limiting both line speed and performance. In addition, the mechanical components limit the relative speed differences between successive layplates. This makes it difficult if not impossible to decouple the operation of the individual layplates to optimize the layplate rotational speeds to achieve the smoothest possible SZ stranding operation.